High Performance of Carbon-Supported Nickel-Platinum Nanocatalysts for Hydrazine Electrooxidation in Alkaline Electrolyte

Hydrazine exhibits many advantages as fuel for alkaline fuel cells. The products from the Direct Hydrazine Fuel Cells (DHFCs) are nitrogen and water that are neither harmful nor contain carbon. Furthermore, the DHFCs theoretical voltage is +1.56 V and DHFCs can operate at near-ambient temperatures (40-80^oC) (1, 2). Pt is an effective electrocatalyst for the hydrazine oxidation reaction (HzOR), as is Ni, but at higher hydrazine concentration and higher temperatures (3-5). In previous works, the combination of Ni with its group metals (Pd and Pt) has been used as electrocatalysts for HzOR at room temperature, but the role of each metal in the composite catalyst remains unclear (4).

In order to investigate the function of each metal in the electrocatalyst toward the HzOR, carbon-supported Ni-Pt nanoparticles were synthesized with 15, 33 and 66 wt% Ni content on Pt/C, using the a thermal decomposition method (6). The materials were characterized by TEM and XRD, and their electrocatalytic activities analyzed by Cyclic Voltammetry and Chronoamperometry in 1.0 mol L^-1 KOH in the presence or absence of 0.1 mol.L^-1 N₂H₄, at 25 ^oC. The diffraction patterns (Figure 1) clearly show the peaks of metallic Pt and the coexistence of two phases: NiO and Ni(OH)_2., without shift in 2-theta of the diffraction peaks. These are evidences that the Ni-oxide phase is segregated from the Pt phase; the electrocatalyst structure seems to be formed by clusters/islands of Ni-oxide on the surface of the Pt/C nanoparticles, instead of Ni-Pt alloys. The average crystallite sizes of Pt resulted in 3.3 nm and the Ni(OH)₂ and NiO crystallite sizes are estimated to be 4.0 nm and 3.4 nm, respectively. Furthermore, there were no significant changes in the average sizes with the increasing of Ni content. The cyclic voltammograms of Figure 2 indicate the following order of activity: 15% > 33% > pure Pt/C > 66% > pure NiO, thus, resulting in an optimal value around 15 wt.% of Ni. These results show an interplay of Ni and Pt atoms for the HzOR, and this seems to be a bifunctional effect, in which the Ni atoms provide OH species for the electrooxidation of the adsorbed hydrogen atoms on the Pt surface, at low potentials.

Acknowledgements: The authors would like to thank FAPESP, CAPES/COFECUB, CNPq and CNRS for financial support.

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